Conventional linear RF (radio frequency) power amplifiers have a problem in that drain efficiency is uniquely reduced when output power is reduced (backed off). To solve the problem, techniques for improving the efficiency at back-off, such as a polar transmitter, EER (envelope elimination and restoration), and ET (envelope tracking), have been developed. In the above techniques, however, when a series variable resistive circuit such as a linear regulator is used in modulating a voltage at a drain (power supply) terminal of the RF power amplifier according to an envelope, the efficiency at back-off is eventually uniquely reduced.
In a conventional method, different power supply voltages are respectively supplied to envelope amplifiers M1 and M2. When the envelope has a small amplitude, the envelope amplifier to which a lower power supply voltage is supplied is used to improve the efficiency. When the envelope has a large amplitude, the envelope amplifier to which a higher power supply voltage is supplied is used to ensure linearity (a class-G modulator).
In the conventional method, however, it is necessary to supply two or more different types of power supply voltages. It is difficult to supply a plurality of power supply voltages from outside in terminal applications with restrictions on costs and mounting areas.
It is also not realistic to generate different power supply voltages from one type of power supply voltage by using a DC-DC converter or the like provided within the amplifier from the viewpoint of mounting areas, costs, and efficiency.